1. Field of the Invention
The present invention relates to a nitride-based light emitting device, and more particularly to a nitride-based light emitting device capable of achieving an enhancement in light emission efficiency and an enhancement in reliability.
2. Discussion of the Related Art
Light emitting diodes (LEDs) are well known as a semiconductor light emitting device which converts current to light, to emit light. Since a red LED using GaAsP compound semiconductors was made commercially available in 1962, it has been used, together with a GaP:N-based green LED, as a light source in electronic apparatuses, for image display.
The wavelength of light emitted from such an LED depends on the semiconductor material used to fabricate the LED. This is because the wavelength of the emitted light depends on the band-gap of the semiconductor material representing energy difference between valence-band electrons and conduction-band electrons.
A gallium nitride (GaN) compound semiconductor has been highlighted in the field of high-power electronic devices including light emitting diodes (LEDs) because it exhibits a high thermal stability and a wide band-gap of 0.8 to 6.2 eV.
One of the reasons why the GaN compound semiconductor has been highlighted is that it is possible to fabricate semiconductor layers capable of emitting green, blue, and white light, using GaN in combination with other elements, for example, indium (In), aluminum (Al), etc.
Thus, it is possible to adjust the wavelength of light to be emitted, in accordance with the characteristics of a specific apparatus, using GaN in combination with other appropriate elements. For example, it is possible to fabricate a blue LED useful for optical recording or a white LED capable of replacing a glow lamp.
By virtue of the above-mentioned advantages of the GaN-based material, techniques associated with GaN-based electro-optic devices have rapidly developed since the GaN-based LEDs became commercially available in 1994.
The brightness or output of an LED manufactured using the above-mentioned GaN-based material mainly depends on the structure of an active layer, the extraction efficiency associated with external extraction of light, the size of the LED chip, the kind and angle of a mold used to assemble a lamp package, the fluorescent material used, etc.
Meanwhile, the reason why it is difficult to grow such a GaN-based semiconductor, as compared to other III-V ground compound semiconductors is that, for example, there is no high-quality substrate such as a wafer made of GaN, InN, AlN, or the like.
For this reason, although the LED structure is grown over a substrate made of a material different from that of the above-mentioned substrate, for example, sapphire, a large amount of defects are generated in this case. Such defects have severe influence on the performance of the LED.
In particular, the active layer, which functions to emit light in the LED structure, has a nitride semiconductor multi-quantum well (MQW) structure. Such an MQW structure includes quantum well layers and quantum barrier layers alternately deposited in a repeated manner. In accordance with this structure, electrons and holes respectively injected from an n-type semiconductor layer and a p-type semiconductor layer are coupled in the quantum well layers, thereby emitting light.
The quantum well layers and quantum barrier layers constituting the above-mentioned MQW structure contain different materials. As a result, stress may be applied to the quantum well layers due to such a material difference.